Solid-state heterojunctions (between p-type and n-type semiconductors) have found widespread application in modern electronics. Such pn heterojunctions typically exhibit diode rectification; they are therefore, useful in a wide variety of electronic circuit applications. The pn heterojunction is useful as a single electronic element, and it is often part of more complex electronic elements such as transistors. A principal feature of the pn heterojunction is the built-in potential at the interface between the p-type (donor) material and the n-type (acceptor) material. This built-in potential arises fundamentally from the different electronegativities of the two materials which make up the heterojunction. The built-in potential and the associated difference in electronegativities is the origin of the rectifying nature of the device. When electrons and holes are photogenerated in the vicinity of the junction, the built-in potential and the associated difference in electronegativities serve to separate the charge. The charge separation at the interface is, therefore, the origin of the photovoltaic effect. Such pn heterojunction diodes can serve as photodiodes and as the fundamental element in a photovoltaic cell, commonly known as a solar cell.
There is extensive prior art on solar cells; such devices are commonly treated in standard texts on semiconductor devices (see for example M. S. Sze, Physics of Semiconductor Devices, Wiley-Interscience, New York, 1981; Chapters 13 and 14). Currently, solar cells are typically fabricated from conventional semiconductors; for example, gallium arsenide, silicon, cadmium sufide etc. Since these materials require costly high temperature processing steps, solar cells made from such materials enjoy limited use. In order to obtain optimum performance from such solar cells, single crystal materials are needed. The growth and the subsequent processing of single crystals is demanding, and therefore even more costly.
In photosynthesis in green plants, the process of charge separation is relatively efficient. There has, therefore, been longstanding interest in striving for a deeper understanding of charge separation in organic systems with the goal of achieving highly efficient charge separation following photo-excitation; see for example Marye Anne Fox and Michel Chanon, Eds., Photoinduced Electron Transfer, Parts A-D, (Elsevier Science Publ., Amsterdam, 1988)
For the above reasons, there has been considerable interest for many years in the development of suitable organic materials for use as the p-type and n-type materials in pn junctions for device application.
The utilization of semiconducting organic polymers (i.e. conjugated polymers) in the fabrication of pn heterojunctions expands the possible applications for conducting polymers into the area of active electronic devices with the possibility of significant cost advantages over existing technology. Controlling the energy gap of the polymer, either through the judicious choice of the conjugated backbone structure or through side-chain functionalization, should make it possible to match the absorption spectrum of the conjugated polymer to the solar spectrum. The ability to make solar cells from uniform polymer layers which have excellent mechanical properties (flexible films with large elongation to break) would enable robust large area devices that could be easily mounted for use.
Typically conjugated polymers are p-type materials in the as-synthesiszed form. Although such semiconducting polymers can be doped n-type (by addition of electrons into the high energy .pi.* electronic energy levels) the resulting known n-type materials are often environmentally unstable.
There is no prior art on the use of conjugated polymers as donors in combination with fullerenes, such as Buckminsterfullerenes, as an acceptor to form donor-acceptor complexes which exhibit photoinduced charge transfer, photoinduced spectral changes (optical memory) and/or which lead to separation of charge and the photovoltaic effect at the interface between the two. There is no prior art using conjugated polymers as the donor layer and fullerenes, such as Buckminsterfullerenes, as the acceptor layer in a pn heterojunction that exhibits the photovoltaic effect.
The fundamental phenomenon underlying the photovoltaic effect is the process of charge separation viewed from the molecular level. A basic description of intramolecular and/or intermolecular photoinduced electron transfer is as follows:
SCHEME 1: PA1 donor (D) and acceptor (A) units are either covalently bound (intramolecular), or spatially close but not covalently bonded (intermolecular); PA1 "1,3" denote singlet or triplet excited states. PA1 (i) Because the semiconducting conjugated polymer (or its precursor polymer) and the fullerenes are soluble, there is no need for heat treatment at elevated temperatures. This greatly simplifies the fabrication procedure and enables a continuous manufacturing process. PA1 (ii) Since the semiconducting polymer layer and the fullerene can be cast onto the substrate directly from solution at room temperature, the device structure may be fabricated on a flexible transparent polymer substrate. Since such polymer films are manufactured as large area continuous films, the use of flexible polymer films as substrate enables the fabrication of large area polymer solar cells using either a batch process or a continuous process.
Step 1: D+A.fwdarw..sup.1,3 D*+A, (excitation on D); PA2 Step 2.sup.: 1,3 D*+A.fwdarw..sup.1,3 (D--A)*, (excitation delocalized on D-A complex); PA2 Step 3.sup.: 1,3 (D--A)*.fwdarw..sup.1,3 (D.sup..delta.+ --A.sup..delta.)*, (charge transfer initiated); PA2 Step 4.sup.: 1,3 (D.sup..delta.+--A.sup..delta.)*.fwdarw..sup.1,3 (D.sup.+.cndot. --A.sup.-.cndot.), (ion radical pair formed); PA2 Step 5.sup.: 1,3 (D.sup.+.cndot. --A.sup.-.cndot.).fwdarw.D.sup.+.cndot. +A.sup.-.cndot., (charge separation);
where:
At each step, the D-A system can relax back to the ground state either by releasing energy to the "lattice" (in the form of heat) or through light emission (provided the radiative transition is allowed). Permanent changes which may occur from ion radical reactions beyond Step 5 are not considered here. The electron transfer (Step 4) describes the formation of an ion radical pair; this does not occur unless EQU I.sub.D* -A.sub.A -U.sub.C &lt;0,
where I.sub.D* is the ionization potential of the excited state (D*) of the donor, A.sub.A is the electron affinity of the acceptor, and U.sub.C is the Coulomb energy of the separated radicals (including polarization effects). Stabilization of the charge separation (Step 5) is difficult; typically the ion-radical pair recombines prior to charge separation so that no current could be delivered to an external circuit.
Thus, the ability to fabricate pn heterojunction diodes from organic materials and in particular from polymers, remains seriously limited.